Noninvasive blood tests are now available to reliably determine paternity as early as the eighth or ninth week of pregnancy.
The New York Times reports that while invasive procedures such as amniocentesis or chorionic villus sampling, performed between 10 and 20 weeks, the new tests—requiring only blood samples from the pregnant woman and potential father—carry less risk of miscarriage. The tests analyze fragments of DNA from the fetus present in the mother's blood. The fetus gets half its DNA from the mother and half from the father.
A paper describing one such test, developed by a company called Ravgen, was published in the New England Journal of Medicine. The cost of blood testing for paternity ranged from $950-$1,650 by Ravgen, compared with around $500 for a conventional postbirth paternity test.
The Times reports that thousands of prenatal tests have been ordered since going on sale.